The work on the organization, expression and control of the mitochondrial genome in HeLa cells will be continued, and will aim at obtaining information on the following questions: a) Mechanism of transcription of mit-DNA and physiological significance of the complete symmetric transcription found in this DNA; b) the metabolism of mitochondrial RNA, with particular reference to the existence of large size precursors of the rRNA species, to the half-life of the two rRNAs and the poly(A)-containing RNAs, to the relationship between poly(A)-containing and poly(A)-lacking RNAs of similar electrophoretic mobility, to the possible occurrence of "wastage" synthesis of mitochondrial RNA; c) the map positions and sequence relationship between the multiple poly(A)-containing RNAs; d) the primary structure of the mit-DNA regions containing the origin of replication and promoter sites for transcription; e) the map positions of the individual specific tRNAs coded for by mit-DNA; f) the specific messenger function of the poly(A)-containing and poly(A)-lacking RNAs coded for by mit-DNA; g) the nature of the proteins synthesized in mitochondria; h) the contribution of the nuclear genome to the synthesis of tRNAs utilized in mitochondria; i) the mechanism of assembly of the multimeric enzyme complex cytochrome c oxidase from its mitochondrially synthesized and cytoplasmically synthesized subunits; j) the nature of the proteins associated with mit-DNA. In another context, the mechanisms controlling the amount of mitochondrial DNA per cell in human cells in culture will be investigated. Furthermore, recently isolated variants of the human cell line VA2-B with altered mitochondrial function will be analyzed as to the site of the mutation and to the nature of the alteration, and additional mutants will be isolated.